Recently, methods for carrying out programming with a laser after fabrication of semiconductor elements, such as memory elements, have been utilized. In selective laser programming methods, there are antifuse-type laser make-link programming, in which two electrically separated conductors are connected to each other using a conductive link, such as a metal, using a laser, and fuse-type laser break-link programming, in which two electrically connected conductors are separated with laser. For example, in case of memory elements, laser break-link programming has been utilized.
When a memory element is found defective upon testing of the memory element after completion of fabrication, a unit cell of the memory element having one or more defective parts is, after identifying the one or more defective parts, replaced with a redundant cell. Then, conductors at the surface of the redundant cell selectively are cut/melted with a laser, thus completing the programming of the redundant cell.
There are various problems with such a laser break-link programming method. For example, the chip should have an increased surface area because of the surface area required for link structures or circuits necessary for the link programing, and also surface damage may result from the programming.
A laser make-link programing method has been suggested, with which the surface area necessary for the link structure can be decreased, and damages to surrounding regions and remainders caused at the time of carrying out laser programing can be prevented.
Such a programing method has been disclosed by Kendal S. Willis in U.S. Pat. No. 4,751,197, in which two conductors are formed with an insulation film between them, with the insulation film broken at a designated part with intensive direction of a laser onto the designated part, thereby connecting the two conductors.
FIGS. 1 and 3 are enlarged plan views illustrating a part of a conventional programmable semiconductor element having an antifuse structure. FIG. 2 is a sectional view along line 2--2 of FIG. 1, and FIG. 4 is a sectional view along line 4--4 of FIG. 3.
Referring to FIGS. 1 and 2, a conventional programmable semiconductor element having an antifuse structure includes insulation film 14 formed on silicon substrate 13, and two conductors 11 and 12, with insulation film 15 between conductors 11 and 12, formed on insulation film 14. In the drawings, reference number 10 represents a link area formed between conductors 11 and 12 and reference member 16 represents a region to which a laser beam will be directed.
Oxide film 15 formed between conductors 11 and 12 can be broken with laser beam 16 focused onto link area 10 due to the resulting heat, which short circuits conductors 11 and 12, electrically connecting them.
Herein, as for the material of the conductors, tungsten (W), aluminum (Al) or a polysilicon film may be used, and oxide film 15 between conductors 11 and 12 is a thermal oxide film having a thickness of about 200 Angstroms. As for the laser, an argon (Ar) ion laser of 0.488 .mu.m wave length or an ND:YAG laser of 1.06 .mu.m wave length can be used, and the size of the laser beam may be 6 .mu.m.
When the power per pulse is 1 micro joule (.mu.j), upon direction of the laser for 20 ms, oxide film 15 having 200 Angstroms thickness can be broken, electrically short circuiting conductors 11 and 12. To accelerate the interface reaction, conductors 11 and 12 may have a voltage of 5 V to 20 V applied thereto, or may be put under hydrogen atmosphere if the material of the conductor is aluminum.
Illustrated in FIGS. 3 and 4 is a programmable semiconductor element having an antifuse structure provided with conductor 33 for linkage formed over two conductors 31 and 32. Oxide film 35 between conductor 33 for linkage and first and second conductors 31 and 32 serves as a layer like oxide film 15 between conductors 11 and 12 of FIGS. 1 and 2.
The semiconductor has first linkage area 30 formed between first conductor 31 and linkage conductor 33, and second linkage area 40 formed between second conductor 32 and linkage conductor 33. Therefore, when laser beam 36 is focused thereon, oxide film 35 of linkage areas 30 and 40 is broken, which short circuits first and second conductors 31 and 32 with linkage conductor 33, connecting first and second conductors 31 and 32, electrically.
Such a programmable semiconductor element having an antifuse structure can have the two conductors connected to each other through linkage conductor 33 even in case a junction between first and second conductors 31 and 32 has not been formed or corrosion has been developed thereon.
However, programmable semiconductor elements having an antifuse structure as illustrated in FIGS. 1 and 3 present certain problems, such as requiring a laser power over a critical level, which is due to the oxide film formed between the conductors in a fixed thickness which is broken in order to short circuit the two conductors by a laser beam directed onto the link areas. In addition, such structures may have a greater contact resistance because the breakage of the oxide film happens not overall but locally, forming non-uniform connections between the conductors.